A display device using organic electroluminescence (EL) has been known as a display device using current-driven light-emitting device. The organic EL display device using the light-emitting organic EL device does not require a backlight necessary for a liquid crystal display device (LCD), and is suitable for reducing thickness of the device. In addition, since there is no limit on the viewing angle either, the organic EL display device is expected to be in practical application as a next-generation display device. In addition, in the organic EL device used for the organic EL display device, the luminance of each of the light-emitting devices is controlled by a current value flowing in the light-emitting device. In that regard, that organic EL device is different from the liquid crystal cell controlled by the voltage applied thereon.
In the organic EL display device, the organic EL devices composing the pixels are usually arranged in a matrix. A passive-matrix organic EL display refers to a display in which organic EL devices are provided at intersections of row electrodes (scanning lines) and column electrodes (data lines), and the organic EL devices are driven by applying a voltage corresponding to a data signal between a selected row electrode and the column electrodes.
There is another display device in which switching thin film transistors (TFT) are provided at intersections of the scanning lines and the data lines, and gates of the drivers are connected to the switching TFT. The switching TFTs are turned on through the selected scanning lines, and the input of the data signals are provided to the drivers. Such a display device in which the organic EL devices are driven by the drivers is referred to as an active-matrix organic EL display device.
The active-matrix organic EL display device is capable of causing the organic EL devices to emit light until next scanning (selection), and is different from the passive-matrix organic EL display device in which the organic EL devices are connected to the row electrode (scanning line) and emit light only when each of the row electrode (scanning line) is selected. Thus, with the active-matrix organic EL display device, the luminance of the display does not decrease even when the number of the scanning lines increases. Accordingly, the active-matrix organic EL display device can be driven with low voltage, and the power consumption can be reduced.
The patent literature 1 discloses a circuit configuration of a pixel unit in an active-matrix organic EL display device.
FIG. 15 is a diagram illustrating the circuit configuration of the pixel and a connection with a circuit around the pixel included in the display device disclosed in the patent literature 1. A display device 100 illustrated in FIG. 15 includes a pixel array unit in which the pixels 100A are arranged in a matrix and a driver unit which drives the pixel array unit. For description purpose, only one pixel 100A configuring the pixel array unit is described in FIG. 15. The pixel array unit includes scanning lines 102 each provided for each row, data lines 101 each provided for each column, the pixels 100A arranged in rows and columns at the intersections of the scanning lines 102 and the data lines 101, and power supply lines 110 each provided for each row. The driver unit includes a horizontal selector 103, a write scanner 104, and a power drive scanner 105.
The write scanner 104 sequentially supplies, to each of the scanning lines 102, a control signal in a horizontal cycle (1H) so as to scan the pixels per row. The power driver scanner 105 supplies a variable power voltage to the power supply lines 110 in synchronization with the line sequential scanning. The horizontal selector 103 switches between the data voltage which is a video signal and a reference voltage in synchronization with the line sequential scanning and supplies the voltage to the data lines 101 in column.
The pixel 100A includes a drive transistor 111, selector transistors 112a and 112b, an organic EL device 113 and a capacitor 114. The selector transistors 112a and 112b are thin film transistors composing a gate group 112. The drive transistor 111 and the organic EL device 113 are connected in series between the power supply line 110 and a reference potential Vcat (for example, the ground potential). With this, the cathode of the organic EL device 113 is connected to the reference potential Vcat, the anode of the organic EL device 113 is connected to the source of the drive transistor 111, and the drain of the drive transistor 111 is connected to the power supply line 110. In addition, the gate of the drive transistor 111 is connected to the first electrode of the capacitor 114, and the other of the source electrode and the drain electrode of the selector transistor 112b. The second electrode of the capacitor 114 is connected to the anode of the organic EL device 113.
Furthermore, the other of the source electrode and the drain electrode of the selector transistor 112a forming the gate group 112 is connected to one of the source electrode and the drain electrode of the selector transistor 112b. The data line 101 and one of the source electrode and the drain electrode of the selector transistor 112a are connected. The gates of the selector transistors 112a and 112 are connected to the scanning line 102.
In the configuration described above, the power drive scanner 105 switches the power supply line 110 from a first voltage (high voltage) to a second voltage (low voltage) with the data line 101 in a threshold detecting voltage. The write scanner 104 raises the voltage of the scanning 102 to “H” level to turn on the selector transistors 112a and 112b, with the data line 101 in the threshold detecting voltage, and applies the threshold detecting voltage to the gate of the drive transistor 111. Subsequently, the power driver scanner 105 switches the voltage of the power supply line 110 from the second voltage to the first voltage such that the capacitor 114 holds the voltage corresponding to the threshold voltage of the drive transistor 111, in a correction period before the voltage of the data line 101 switches from the threshold detecting voltage to the data voltage. Next, the write scanner 104 changes the voltages at the selector transistors 112a and 112b to “H” level such that the capacitor 114 holds the data voltage. To put it differently, the data voltage is added to a voltage corresponding to the threshold voltage of the drive transistor 111 that has been held, and is written on the capacitor 114. Subsequently, the drive transistor 111 receives a current supply from the power supply line 110 in the first voltage and causes the flow of the driving current according to the voltage that is held flows in the organic EL device 113.
As described above, the write scanner 104 writes and holds the data voltage by turning the gate group 112 on and off. The configuration of the gate group 112 in which the two selector transistors are connected in series is referred to as a double-gate structure. With the double-gate structure, the turn-off resistance of the gate group 112 is doubled, and even when one of the selector transistors causes off-leakage, the off-leakage is suppressed by the other selector transistor, reducing the off-leakage current to approximately half.
According to the patent literature 1, the double gate structure allows writing precise luminance information on the pixel, and provides a display device which has high-image quality and does not cause variation in the luminance of the organic EL device 113.